Air flow control in shaft furnaces

ABSTRACT

Apparatus and method of controlling and distributing air in a vertical shaft furnace for indurating oxide pellets including combustion chambers for introducing heated air at opposite sides of a furnace and varying the quantity of air supplied at one side of the furnace from a minimum to a maximum quantity and simultaneously varying the air delivered at the opposite side from a maximum to a minimum quantity.

United States Patent [191 111 3,756,768

Escott Sept. 4, 1973 [54] AIR FLOW CONTROL IN SHAFT FURNACES 3,645,5142/1972 Lado 432/17 [75] Inventor: Robert M. Escott, Toledo, Ohio [73]Assignees: Midland-Ross Corporation; Erie 'i' Q mi! :l9h n LQHbY .v

Development Company both of Attorney-Malcolm R. McKmnon, Irvin L. Grohet Cleveland, Ohio [22] Filed: May 11, 1972 1 [57] ABSTRACT [2]] Appl.No.: 252,378

Apparatus and method of controlling and distributing air in a verticalshaft furnace for indurating oxide pel- [2%] :J.S.((:3ll. 432/17, 432/79lets including combustion chambers for introducing 5 f F27! 1/10 heatedair at opposite sides of a furnace and varying the I 1 0 can: 5 quantityof air supplied at one side of the furnace from 432/78 101 a minimum toa maximum quantity and simultaneously varying the air delivered at theopposite side from a [56] uNlTE g gfizfr gs gz rENTs maximum to aminimum quantity.

2,739,800 3/1956 Sisco 432/79 X 11 Claims, 2 Drawing FiguresPAIENTEDSsr'mn sum 1 or 2 l'rseJea PAIENTED SE? 4193- snmaurz 1 AIR FLOWCONTROL IN SHAFT FURNACES SUMMARY OF THE INVENTION This inventionrelates to the indurating of pellets of material in a shaft typeindurating furnace and, more particularly, to a method and apparatus forcontrolling the distribution of air in the furnace to maintain uniformindurating temperatures, particularly at the upper end of the furnace.

In the indurating of materials in a shaft furnace and particularlypellets of iron ore, finely ground and bailed iron ore concentrates inthe form of balls or pellets are introduced in a cold state into the topof a. vertically disposed furnace after which the pellets are heated toan indurating temperature and are permitted to move downwardly in thefurnace under the influence of gravity. At the same time, air underpressure is introduced at lower levels of the vertical furnace to flowupwardly and countercurrently to the mass of downwardly moving, hotpellets. The air serves to cool the pellets so that they can be handledat discharge at the bottom of the furnace and, at the same time, the airabsorbs the heat from the pellets as it flows upwardly. Heat for theindurating process also comes from the exothermic reaction resultingfrom the magnetite in the pellets which, when oxidized, give off heatthat is captured by the upwardly flowing air. Also, it is usual to addair in a preheated state near the upper end of the vertical furnace. Theheat for the indurating process, that is, the heat resulting from theexothermic reaction, the heat recuperated from the downwardly moving hotpellets by the upwardly moving air and the heat which is added near theupper levels of the furnace should have a uniform distribution,particularly at the indurating level of the furnace if pellets ofuniform quality are to result from the process.

The problem of insuring uniform thermal treatment of the green pelletswhich are introduced at the top of the furnace is aggravated by a numberof variables which cause variations in the temperature at differentlocations in a given level of the furnace. The upwardly flowing airtends to seek the coolest locations for flow which results inchanneling, that is, the flow of cooler air in one location and warmerair in another, adjacent location. This condition is often initiated andaggravated by the variations in the permeability of the bed of pelletsat a given level of the furnace due to variations in pellet size oraccumulations of chips or fines of material. Such variables: result inirregular flow and temperatures of the upwardly flowing air and once theirregular condition is initiated, it becomes more aggravated since thecold air will tend to flow to colder locations and become even colder.This effect is referred to as channeling which results in cold spots andhot spots at the indurating level and is a condition which must beminimized if a good indurated oxide pellet product is to be produced.

Although channeling could be avoided and the indurating processsuccessfully carried out by introducing all of the air requirements inheated form, such operation would be commercially impractical because ofthe uneconomical use of fuel and because additional, expensive equipmentwould be required to cool the pellets for handling. It has been thepractice to reduce the amount of heated air as much as possible and tointroduce the balance of the air in a cold state at the bottom of thefurnace. It has been found, however, that a reduction of the quantity ofheated air much below onethird of the total air required in the processresults in the undesirable channeling of air and irregular temperaturesacross the indurating bed at the upper end of the furnace.

It is an object of the invention to provide a method and apparatusforintroducing preheated air into a vertical shaft, indurating furnace insuch a manner that channel flow of air is inhibited to make for a moreuniform temperature at the upper levels of the furnace where induratingtakes place.

Another object of the invention is to provide a method and apparatus bywhich preheated air is introduced to the furnace in such a manner thatthe formation of channels of cold and hot air is greatly reduced, makingit possible to use less heated air which results in fuel savings, andmore cold air which results in a greater cooling of the pellets so thatthey can be more easily handled upon removal at the bottom of thefurnace.

It is still another object of the invention to provide a method andapparatus by which preheated air is introduced into a vertical shaftfurnace in such a manner that a greater portion ofthe mass of pelletscomes under the influence of preheated air.

In accordance with the present invention, a predetermined quantity ofair for a given period of time is introduced at an upper level in avertical shaft indurating furnace. A portion of the air is introduced atone location and is varied over a predetermined period of time from aminimum volume to a maximum volume and in the next predetermined periodof time, from a maximum to a minimum. At the same time, the balance ofthe preheated air is introduced at other locations at substantially thesame level of the furnace and is varied in a cycle varying from amaximum to a minimum and back to a maximum so that the total quantity ofair introduced in the heated form remains constant. The cycle isrepeated continuously to influence a greater portion of thecross-section of the furnace with preheated air to deter the formationof channels of air at different temperatures.

BRIEF DESCRIPTION OF THE, DRAWINGS FIG. 1 is a perspective,cross-section view of a vertical shaft furnace with parts broken away,and

FIG. 2 is a schematic representation in flow sheet form of apparatusoperable for use in the indurating process of the present invention.

. DETAILED DESCRIPTION Referring to the drawings, a vertical shafttypeindurating furnace of a type well known in the art is generallydesignated 10. The furnace is generally rectangular in horizontalcross-section and'm'ay be'considered as having two separateverticalsections made up of an upper heating stove l2 and a lowercooling stove 14. At the upper end of the furnace, a stock linefeeder'l6 is located which is translatable horizontally and controlledin a well known manner to deposit a layer of green, untreated pellets onthe stock level or line indicated at 18 in FIG. 2. The pellets may be inthe form of bailed magnetite and binder and the function of the stockline feeder 16 is to uniformly distribute the green balls over theentire stock line area and to automatically correct the level of the bedof pellets in response to controls which are not shown but which alsoare well known in the art.

The pellets flow downwardly by gravity and are discharged at the bottomof the furnace through discharge pipes 22, best seen in FIG. 1. Aplurality of agitating shafts 24 are disposed horizontally above thedischarge pipes 22 and are agitated by means of links 26 at apredetermined rate to control and to assist the downward movement of theindurated pellets.

Air under pressure from a suitable source indicated at 27 in FIG. 2 isfed to the interior of the furnace through louvers 28 only one of whichis shown. it should vbe understood, however, that a plurality of saidlouvers may be used to distribute air across the lower level of thefurnace 10. The air is under a slight pressure, to the order of 7 psi,and tends to move upwardly in the furnace because of the constriction toair flow offered by the pipes 22 at the bottom of the furnace. The airis introduced to the furnace in a cold condition and is heated as itflows upwardly by contact with the downwardly gravitating hot pelletswhich, in turn, are being cooled by the upwardly moving air.

As best seen in FIG. 2, additional air is introduced to the furnace 10in a heated condition and at a pressure to the order of 4 psi. Theheated air is introduced at an upper portion of the furnace 10 betweenthe heating stove l2 and the cooling stove 14 and at opposite sides ofthe furnace from combustion chambers 36 and 38. Each of the combustionchambers 36, 38 communicates with the furnace 10 through separatemanifold arrangements 40 to a plurality of hot air ports 41 which can beseen in FIG. 1 and which are located around the perimeter of thefurnace. The ports 41 in one half of the interior perimeter of thefurnace are supplied by air from the combustion chamber 36 and the otherone half of the ports 41 in the perimeter of the furnace are supplied bythe combustion chamber 38. As seen in FIG. 2, air is fed to burners 42and 44 at the bottom of the combustion chambers 36 and 38, respectively,by conduits 46 and 48 communicating with sources of air such as blowers50 and 52. Fuel to the burners 42 and 44 is supplied from suitablesources of fuel 54 and 56 by way of conduits 58 and 60, respectively.The combustion chambers 36 and 38 serve primarily to make up the thermallosses in the indurating process and provide a means for controllingtemperature distribution, particularly at the upper end of the furnace10 near the stock line level 18.

. During normal operation of an indurating pellet furnace, the residencetime of the pellets within the furnace is to the order of 5 to 6 hoursand it has been the usual practice to introduce approximately one-thirdof the air in heated form at the upper section of the furnace and theremaining two-thirds of the air through the cold air louvers 28.

Heat for the indurating process is derived from the heat exchangebetween the downwardly gravitating pellets and the countercurrentlyflowing air which tends to cool the pellets and at the same time absorbsthe heat and transfers it upwardly. Additionally, hot air is suppliedthrough the combustion chambers 36 and 38 and a third source of heat isderived from the rapid oxidation of the magnetite in the pellets. Theexothermic reaction results in heat which is absorbed by the upwardflowing air to increase the temperature of pellets disposed above.

During actual operation, the temperatures of the pellets and the airwithin the furnace are exceedingly high, particularly near the top ofthe pellet bed and just below the stock line 18. By way of example,pellets are discharged from the pipes 22 at a relatively cooltemperature which may be to the order of 500 to 600 F. which issufficiently low for further handling of the pellets by conveyors or thelike. The temperature at the upper portion of the cooling stove l4 andbelow the hot air manifolds 40 and ports 41, however, may have attaineda temperature to the order of 2,300 F. At a level of l or 2 feet belowthe stock line 18, the temperature may be to the order of 2,500 F. it isat this level that induration of the pellets is initiated and the airgives up its heat to the pellets so that the air is discharged from thetop of the furnace at a temperature to the order of 500 or 600 or less.

During the upward flow of air, it is desirable to maintain as uniform anair-pellet temperature as possible throughout the horizontalcross-section of the furnace at any given level. However, it has beenfound that variations in pellet density, and heat losses through thewalls of the furnace l0 create locations which may be come cooler thanother locations in a particular crosssectional level of the furnacecausing a nonuniform distribution in the air-pellet temperature. Thetendency of the upwardly flowing air is to seek the coolest of theseareas and once this phenomenon is initiated, the cool flowing air tendsto cool the pellets even more and thereby attract even more cold air.This results in channeling of the upward flowing air by which is meantthat shafts of relatively cool and relatively hot air are disposedadjacent'to each other and result in what may be terrnedcold spots orhot spots at the indurating level immediately below the stock line level18 at the upper end of the furnace. The hot spots result in excessiveheat which fuses the pellets together and interferes with properoperation of the furnace and the cold spots result in improperinduration of the pellets.

As pointed out before, it has been usual to introduce approximatelytwo-thirds of the air required for the indurating process through thecold air louvers 28 and approximately one-third of the air is introducedfrom the combustion chambers 36 and 38. Of the heated air, it has beenusual to introduce one-half from the combustion chamber 36 and theremaining one-half from the combustion chamber 38. A portion of theshaft air on its way to the stock line 18 eventually crosses thedescending pellets which have been heated by the combustion air tothereby raise the temperature of the air to the desired degree.

In the method of the present invention the total combustion or heatedair which is introduced into the furnace is maintained at a constantquantity for any given period of time but the amount supplied by each ofthe combustion chambers 36 and 38 is varied so that an increase in theamount of air from one of the combustion chambers during a given periodof time is accompanied by an identical decrease in quantity during thatsame period of time from the other of the combustion chambers. Assumingthat 35 percent of the total air required for the indurating process isto be supplied by the com bustion chambers 36 and 38, and furtherassuming that the minimum amount of air to be supplied by any one of thecombustion chambers is to be 7 percent, when the combustion chamber 36is supplying, for example, 7 percent of the total air, the remainingamount,

namely 28 percent, is being supplied by the other of the combustionchambers 38. As the percentage of air from chamber 36 is increased to 28percent and then decreased back to 7 percent, the amount of air from theother combustion chamber 38 is simultaneously decreased from 28 to 7percent and then back to 28 percent thereby always maintaining the totalquantity of heated air which is being introduced to the furnace at aconstant. The air being supplied from the combustion chambers 36 and 38is continuously cycled in this manner over a predetermined period oftime, for example, a A hour period in order to provide ample timefor thevarious time controlled equipment to maintain the proper air-fuel ratioin the burners.

It should be understood, also, that the amount of 7 percent which hasbeen referred to as a lower limit of the quantity of air supplied by thecombustion chambers 36 and 38 is by way of example only and that thepercentage could be more or less than 7 percent. The amount of air,however, should not be lower than that required to maintain goodcombustion in the combustion chambers and proper heating of the pelletsdisposed adjacent to the furnace wall in the area of the outlet of thecombustion chambers.

The apparatus employed to control the flow of heated air from thechambers 36 and 38 through the manifolds 40 is illustrated in the flowdiagram shown in FIG. 2. A signal generator 62 delivers a signal'to thecontrol system associated with the combustion chamber 36 and another,separate signal to the control system associated with the' combustionchamber 38. In the embodiment shown,one of the signals varies between aminimum and a maximum value and, at the same time, the other signaldecreased from a maximum to a minimum.

Referring to the control system associated with the combustion chamber36, the signal from the signal generator 42 is delivered to a flowcontroller 64 which is known in the art as a cascade flow controller.Such controllers sense the rate of air flow in a conduit 46 between thesource 50 and burner 42 and deliver a signal to a motor operated controlvalve 66 to vary the volume of air from the source 50 in accordance witha supplied signal received from the signal generator 62. As the signalvaries so too does the quantity of air being delivered.

Also employed in the control system associated'with the combustionchamber 36 is a temperature controlling instrument 68 which responds tothe temperature in the combustion chamber 36 that is sensed by a'thermocouple indicated at 70 to vary the setting of a fuel valve 72 inthe line 58, between the fuel supply 54 and the burner 42. As the volumeof air being delivered to the burner 42 changes, the volume of fuel willalso change accordingly in order to maintain the temperature of theheated air at the level desired as determined by the temperaturecontrolling instrument 68. l

The control system which is associated with the combustion chamber 38 isidentical to that associated with the combustion chamber 36 and utilizesa flow controller 74 to regulate an air valve 76 and a temperaturecontroller 78 responding to a thermocouple 80 to regulate a fuel valve82. The flow controller 74, however, is responsive to a different signalthan the flow controller 64. Since one signal is increasing as the otheris decreasing, the combined volume of air delivered by the combustionchambers 36 and 38 remains constant but the quantity delivered by one ofthe combustion chambers is decreasing as the volume delivered by theother of the combustion chambers is increasing. As the quantities of airdelivered by the combustion chambers vary, the temperature is maintainedat a substantially constant value by the control of fuel suppliedthrough the valves 72 and 82.

The stock line 18 at the upper end of the furnace is under the influenceof cool air and preheated air in approximately the same ratio as therespective quantities of air that are introduced to the furnace. Forexample, in a conventional vertical shaft furnace let it be assumed thatpercent of the total air is cool air introduced at the bottom of thefurnace and the remaining 35 percent is heated air introduced in equalamounts from opposite sides of the furnace and at a.higher elevation inthe furnace. Under such conditions, approximately 17 and r: percent ofthe distance of the stock line from each side of the furnace is underthe influence of preheated air and the remaining 65 percent of the stockline located centrally of the furnace is. under the influence of theshaft air. In the present method and apparatus in which the preheatedair is varied between a minimum and a maximum at one side of the furnacewhile the amount is varied from a maximum to a minimum at the other sideof the furnace, the same percentages of cool and preheated air as in theprior art result in a greater percentage of the stock line being underthe influence of preheated air. In the example previously given whereinthe preheated air is varied between 7 and 28 percent of the total airburden, it can be seen i that more than one-half of the stock line comesunder the influence of preheated air during succeeding periods of time.To be more specific, the total percentage of the stock line beingaffected by preheated air is approximately 56 percent during a fullcycle in which each of the combustion chambers 36 and 38 has deliveredits maximum portion of the heated air.

,As the pellets which have been heated by the combustion air introducedadjacent the opposite walls of the furnace descend through the lowerstove 14, the cold pellet paths which may have been formed are disturbedand obliterated and the channeling of cold air will be eliminated tomake for a uniform and desirable temperature level at the horizontalcross-section of the indurating or stock line level. The variouspercentages and linear lengths which have been used by way of examplemay be considered to be one condition of operation. However, it shouldbe understood that operation of the combustion chambers 36 or 38 in themanner described has the desirable effect of reducing channeling andmaking for a more uniform temperature at the stock line which makes itpossible to increase the total amount of cool air supplied' through thelouvers 28 at the bottom of the cooling stove l4 and, in likemanner,reduce the percentage of hot air supplied byway of the combustionchambers. This results in a decrease in the amount of fuel required toheat the combustion air and, at the same time, results in reducing thetemperature of the pellets being discharged through the pipes 32 so thatthey may be handled more conveniently and economically.

sults in a proportional savings in fuel which, at the rate at which fuelis utilized, results in substantial savings which is of great importanceto the commercial utilization of the process. Because of theeffectiveness of the method of the present invention in minimizingchanneling of the process air, the reduction of the percentage ofcombustion air required may be to as much as 20 percent of the totalair, thereby resulting in even greater fuel savings.

Although the process and apparatus has been described in terms of theinduration of pellets of iron ore materials, it should be understoodthat any material developing an exothermic reaction under the influenceof a heated atmosphere could be utilized in the performing of theprocess. Moreover, the apparatus and process which has been describedhas been discussed in terms of using air in both cold and heated form.It should be understood, however, that with certain materials otherforms of a heat exchange gas might be desirable.

It will now be seen that there has now been provided an apparatus andmethod by which the air is controlled and distributed in a shaft furnacein such a manner that a greater portion of the pellet bed comes underthe influence of heated air than in prior art forms of furnace andprocess and, as a consequence, the undesirable channeling of air isminimized so that the amount of heated air can be reduced therebyresulting in fuel economies and, at the same time, the amount of coldair may be increased to better cool the pellets for more easy removal atthe discharge end of the furnace.

What is claimed is:

1. In a method of indurating pellet like material which comprisesflowing the material downwardly in a shaft furnace and causing heatexchange gas to flow upwardly through the furnace relative to saidmaterial, the improvement comprising heating a portion of said heatexchange gas introduced to said furnace, introducing a predeterminedvolume of said heated gas to said furnace over a predetermined period oftime, introducing said heated gas to at least two separate locations ata given level in said furnace, said heated gas being introduced at oneof said locations being varied in volume during said predeterminedperiod of time between a minimum and a maximum.

2. The method according to claim 1 wherein said heated gas is introducedto locations at opposite sides of said furnace.

3. The method according to claim 1 wherein said heated gas introduced atsaid one of said locations is distributed to one-half of the perimeterof said furnace.

4. In a method of indurating pellet like material which comprisesflowing the material downwardly in a vertical shaft furnace and causingheat exchange gas to flow upwardly through said furnace relative to saidmaterial, the improvement comprising heating a predetermined percentageof said heat exchange gas prior to introduction into said furnace,introducing a part of said fixed percentage of heated gas at anintermediate vertical level and at one side of said furnace, introducingthe remaining part of said fixed percentage of heated gas at theopposite side of said furnace, said first mentioned part of said heatedgas being varied from a minimum to a maximum amount over a predeterminedperiod of time, and simultaneously varying said remaining part of saidheated air from a maximum to a minimum amount during said predeterminedperiod of time.

5. The method according to claim 4 wherein said heated gas is maintainedat a predetermined, uniform temperature during the said predeterminedperiod of time that said parts of said fixed percentage of heated gasare introduced to said furnace.

6. The method according to claim 4 wherein said first mentioned part ofsaid heated gas introduced to one side of said furnace is distributed toone-half of the perimeter of said furnace and said remaining part ofsaid heated gas is distributed to the remaining perimeter of saidfurnace at the other side of said furnace.

7. In a method of indurating pellet like material in a shaft typefurnace comprising establishing a gravitationally descending mass ofpellet like material in initially unheated form, removing induratedpellets from the bottom of said mass and replacing pellets in unheatedform at the top of said mass to maintain the height of the latter,continuously introducing heat exchanged gas in an unheated state to thebottom portion of said mass of pellets, simultaneously introducing heatexchanged gas in a heated state to an intermediate level and at oppositesides of said mass, said heated and unheated heat exchange gas beingmaintained at a predeterrnined substantially constant volume for a givenperiod of time, a portion of said heated heat exchange gas beingintroduced at one side of said mass and being varied in volume from apredetermined minimum up to a maximum and back to a minimum during saidgiven period of time and the remaining portion of said heated heatexchange gas being introduced at the other side of said mass and beingvaried in volume from a predetermined maximum down to a minimum and backto a maximum during said given period of n'me.

8. The method of claim 7 in which the volume of said heated heatexchange gas introduced during said given period of time issubstantially lower than the volume of said unheated heat exchange gas.

9. The combination set forth in claim 8 in which said heated heatexchange gas is between 20 and 35 percent of said predetenninedsubstantially constant volume of heat exchange gas.

10. Apparatus for distributing heat exchange gas to a bed of pellet likematerial in a shaft furnace comprising, in combination, a shaft typeindurating furnace comprising an upper heating stove and a lower coolingstove, first and second sources of heated gas, first and second manifoldmeans operatively connecting said first and second sources of heated gaswith separate portions of said furnace intermediate said upper and saidlower stoves, respectively, control means operatively associated withsaid first manifold means and 0perative to vary the quantity of heatedgas between a minimum and a maximum quantity, second control meansoperatively associated with said second manifold means and operative tovary the quantity of heated gas between a maximum and a minimum ininverse proportion to the quantity of heated gas introduced through saidfirst manifold means, said first and second control means beingoperatively associated with each other to maintain the quantity of saidheated gas delivered through said first and second manifold meansconstant.

11. The combination set forth in claim 10 in which said heated gas isintroduced at super atmospheric pressure.

* i i i

1. In a method of indurating pellet like material which comprisesflowing the material downwardly in a shaft furnace and causing heatexchange gas to flow upwardly through the furnace relative to saidmaterial, the improvement comprising heating a portion of said heatexchange gas introduced to said furnace, introducing a predeterminedvolume of said heated gas to said furnace over a predetermined period oftime, introducing said heated gas to at least two separate locations ata given level in said furnace, said heated gas being introduced at oneof said locations being varied in volume during said predeterminedperiod of time between a minImum and a maximum.
 2. The method accordingto claim 1 wherein said heated gas is introduced to locations atopposite sides of said furnace.
 3. The method according to claim 1wherein said heated gas introduced at said one of said locations isdistributed to one-half of the perimeter of said furnace.
 4. In a methodof indurating pellet like material which comprises flowing the materialdownwardly in a vertical shaft furnace and causing heat exchange gas toflow upwardly through said furnace relative to said material, theimprovement comprising heating a predetermined percentage of said heatexchange gas prior to introduction into said furnace, introducing a partof said fixed percentage of heated gas at an intermediate vertical leveland at one side of said furnace, introducing the remaining part of saidfixed percentage of heated gas at the opposite side of said furnace,said first mentioned part of said heated gas being varied from a minimumto a maximum amount over a predetermined period of time, andsimultaneously varying said remaining part of said heated air from amaximum to a minimum amount during said predetermined period of time. 5.The method according to claim 4 wherein said heated gas is maintained ata predetermined, uniform temperature during the said predeterminedperiod of time that said parts of said fixed percentage of heated gasare introduced to said furnace.
 6. The method according to claim 4wherein said first mentioned part of said heated gas introduced to oneside of said furnace is distributed to one-half of the perimeter of saidfurnace and said remaining part of said heated gas is distributed to theremaining perimeter of said furnace at the other side of said furnace.7. In a method of indurating pellet like material in a shaft typefurnace comprising establishing a gravitationally descending mass ofpellet like material in initially unheated form, removing induratedpellets from the bottom of said mass and replacing pellets in unheatedform at the top of said mass to maintain the height of the latter,continuously introducing heat exchanged gas in an unheated state to thebottom portion of said mass of pellets, simultaneously introducing heatexchanged gas in a heated state to an intermediate level and at oppositesides of said mass, said heated and unheated heat exchange gas beingmaintained at a predetermined substantially constant volume for a givenperiod of time, a portion of said heated heat exchange gas beingintroduced at one side of said mass and being varied in volume from apredetermined minimum up to a maximum and back to a minimum during saidgiven period of time and the remaining portion of said heated heatexchange gas being introduced at the other side of said mass and beingvaried in volume from a predetermined maximum down to a minimum and backto a maximum during said given period of time.
 8. The method of claim 7in which the volume of said heated heat exchange gas introduced duringsaid given period of time is substantially lower than the volume of saidunheated heat exchange gas.
 9. The combination set forth in claim 8 inwhich said heated heat exchange gas is between 20 and 35 percent of saidpredetermined substantially constant volume of heat exchange gas. 10.Apparatus for distributing heat exchange gas to a bed of pellet likematerial in a shaft furnace comprising, in combination, a shaft typeindurating furnace comprising an upper heating stove and a lower coolingstove, first and second sources of heated gas, first and second manifoldmeans operatively connecting said first and second sources of heated gaswith separate portions of said furnace intermediate said upper and saidlower stoves, respectively, control means operatively associated withsaid first manifold means and operative to vary the quantity of heatedgas between a minimum and a maximum quantity, second control meansoperatively associated with said second manifold means and operative tovary the quantity of heated Gas between a maximum and a minimum ininverse proportion to the quantity of heated gas introduced through saidfirst manifold means, said first and second control means beingoperatively associated with each other to maintain the quantity of saidheated gas delivered through said first and second manifold meansconstant.
 11. The combination set forth in claim 10 in which said heatedgas is introduced at super atmospheric pressure.